1. Technical Field
The invention described herein relates generally to material testing devices, and, in particular, to a material testing device which provides very high rates of axial and radial loadings simultaneously on a specimen being tested.
2. Background Art
The mechanical properties of a material specimen may vary under various combinations of axial and radial loads in tension, compression, and/or shear, depending on the particular type and combination of loads and the rate at which these loads are applied to the material specimen.
The rate of applied loads is usually dealt with by designers on a relatively simplistic basis. It is known that dynamics of loadings affects the stress in a given item and various techniques exist for approximating corrective factors in stress calculations for these differences. For example, the well known Barth equation .sigma..alpha.=.sigma.1200/(1200+v), where .sigma..alpha. is the design stress of a gear tooth at a pitch velocity v and .sigma. is a safe static stress for that tooth, is based on an English rule published in 1869. Likewise, the equation for the shock factor b, b=(1+(1+h/y).LAMBDA.(0.5)), where h is the height to produce a given velocity of impact and y is the formation produced by a static load of equal magnitude, shows that for an impact, h=0, a factor of 2 is determined to be multiplied to the static load.
These calculations do not include any correction for the response of a given material to dynamic loading. This is partially due to the insensitivity of most material properties to relatively small changes in rates of loading. However, at a very high rate of loading such as that experienced in the field of ballistics a material's response to loading can be quite different from loads of the same magnitude applied at a lower rate or applied for a different duration. Nylon, used in rotating bands for various artillery shells should not work according to the material properties provided by the handbooks, which has data based on static loading. That these bands do work indicates that nylon must behave differently at the high rates of loading in the ballistic application, at least for the duration to which they are subjected to these loads. The properties of nylon have not been documented at these rates of loading.
Also, it is well known that combinations of loads create a so called effective stress which can be quite different from any of the individual applied stresses. Various theories are available to estimate this effective stress for cases with combined loading. Most are conservative which leads to inefficient designing.
Maximum normal stress theory states states that failure occurs when the largest principal stress equals the yield strength, .sigma.=-s.sub.y.
Maximum shear stress is used for ductile material only and states that yielding occurs in shear loading at a magnitude of half the yield stress in tension.
The Von Mises-Hincky theory evolved from observation that when ductile materials were stress hydrostatically their yield strengths were much higher than their yield strengths in simple tension tests. The theory states that the Von-Mises stress .sigma. is related in a quadradic manner to the principal stresses .sigma..sub.1, .sigma..sub.2, .sigma.=(.sigma..sub.1.sup.2 -.sigma..sub.1 .sigma..sub.2 +.sigma..sub.2.sup.2).LAMBDA.(0.5).
The ability to characterize material properties for specific combinations of loads at ballistic rates of loading would greatly enhance the designer's ability to generate effective and efficient designs.
There are many material testing devices known to the art. However, none of the known devices for hydrostatically loading test samples provide rates of loading which are required for dynamic characterization. Also, none of the known high rate axial loading devices provide the biaxial and hydrodynamic information required for a complete material characterization.